1. Field of the Invention
The present invention relates to overhead projectors of the so-called reflective type wherein the light source and projection lens are contained in a projection head supported above a stage on which may be located a transparency to be imaged on a distant vertical surface.
2. Description of the Prior Art
U.S. Pat. No. 3,293,982, assigned to the assignee of the present invention, describes a reflective overhead projector comprising a light source and a projection lens supported above an unsymmetrical reflective stage upon which may be placed a transparency. A transparency is usually in the form of a thin sheet or plate of transparent material on which an image is formed with colored transparent dyes, opaque inks or surface etching. A beam of light is directed from the light source in the projection head, through the transparency to the reflecting surface of the stage. The light reflected by the reflecting surface of the stage again passes through the transparency to a projection lens also located in the projection head and finally directed to, and focused on, a distant, substantially vertical screen.
Existing reflective overhead projectors utilize a single lamp in the projection head as the light source. Thus, if the lamp burns out, the presentation in progress must be halted and the lamp replaced. This situation has resulted in less than total acceptance of reflective overhead projectors, because users are not confident that a presentation will proceed uninterrupted. Thus, despite
have tended to prefer transmissive projectors which contain the light source below the stage supporting the transparency. These transmissive projectors have room for multiple lamps which may be alternately energized in the event the lamp in use burns out. This is usually accomplished by operating a slide which carries the burned-out lamp from the system optical axis and carries a fresh lamp into its place. The fresh lamp is automatically energized when it reaches the illumination position. By utilizing this arrangement, the user need only operate the slide in the event of lamp burn-out to continue with his or her presentation.
It would be desirable to supply a reflective overhead projector with an alternate lamp to enable the same sort of worry-free arrangement whereby the lamp may be quickly replaced in the event of burn-out. Heretofore, however, this has not been accomplished because the projection head would be enlarged to an unacceptable degree.